Atmospheric wind observations with passive optical remote sensing techniques that measure Doppler shift have a long heritage. To date, space based optical measurements of winds in the Earth's atmosphere have been performed using either Fabry-Perot interferometers or Michelson interferometers. Both instrument types use a limb viewing geometry to detect the Doppler shift of discrete atmospheric emission lines caused by the bulk velocity along the line of sight at the tangent layer. The horizontal wind vector is determined by combining two measurements of the same air mass with orthogonal look direction, typically taken several minutes apart, 45° and 135° from the velocity vector of the satellite.
Fabry-Perot Heritage
As discussed in Hayes P. B. et al., “The High-Resolution Doppler Imager on the Upper Atmosphere Research Satellite”, J. Geophys. Res., 98, 10713-10723, 1993, and Killeen T. L. et al., “TIMED Doppler Interferometer (TIDI), Proc. SPIE, 3756, 289-315, 1999, the High-Resolution Doppler Imager (HRDI) on NASA's Upper Atmospheric Research Satellite (UARS) and TIDI on NASA's Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) mission utilize a triple and a single Fabry-Perot interferometer, respectively, to measure emissions between 550-900 nm. The Fabry-Perot instruments utilize one or multiple etalons in series to isolate and spectrally resolve the emission line(s) of interest. The spectrum over a narrow wavelength range is obtained directly by imaging the ring pattern produced by the interferometer on a position sensitive detector. Once the spectrum is obtained, the wind speed can be derived from the line position. The temperature can be determined from either the line width or a line ratio. The biggest technical challenge for the Fabry-Perots lies in achieving the required etalon alignment tolerances (better than ˜λ/20) and maintaining this alignment during flight. Although many resolution elements are measured in parallel, the solid angle Ω for a single resolution element is determined by the resolving power R (i.e. Ω=2π/R) which can be small at the resolution required for Doppler measurements. Since the high resolving power necessitates a small solid angle, a large interferometer aperture may be required to obtain adequate signal on faint emissions. This results in a larger, heavier instrument.
Stepped Michelson Heritage
As discussed in Shepherd et al., “WINDII, the Wind Imaging Interferometer on the Upper Atmosphere Research Satellite”, J. Geophys. Res., 98, 10725-10750, 1993 (“Shepherd et al.”), the Wind Imaging Interferometer (WINDII) on UARS uses an all-glass, field widened, chromatically, and thermally compensated, phase-stepped Michelson interferometer (also termed Stepped Fourier Transform Spectrometer or stepped FTS). Several other versions of phase-stepped interferometers have been built or proposed for the measurement of telluric winds (see Babcock et al., “A Prototype Near-IR Mesospheric Imaging Michelson Interferometer (MIMI) for Atmospheric Wind Measurement,” Eos Trans. AGU, 85(47), Fall Meet. Suppl., Abstract SA41A-1040, 2004, and Ward et al., “The Waves Michelson Interferometer: A visible/near IR interferometer for observing middle atmosphere dynamics and constituents,” Proc. SPIE Int. Soc. Opt. Eng., 4540, 100, 2001. (“Ward et al. 1”)) and winds on Mars (see Ward, W. E. et al., “An imaging interferometer for satellite observation of wind and temperature on Mars, the Dynamics Atmosphere Mars Observer (DYNAMO),” Proc. SPIE Int. Soc. Opt. Eng., 4833, 226, 2002 (“Ward et al. 2”)).
The basic principle behind all phase stepped Michelson interferometers is to measure a minimum of three, but typically four, interferogram points of a single isolated atmospheric emission line. The phase points are spaced by ˜λ/4 (90°) about a step (or offset) in optical path difference (OPD) that is large enough to be sufficiently sensitive to both wind speed, which results in a phase shift at high OPD, and temperature, which results in a variation in modulation depth. This principle is illustrated in FIG. 1. It shows a schematic interferogram as it would be recorded by a conventional scanning Michelson interferometer viewing an isolated, single Gaussian (temperature broadened) emission line. Zero path difference is at the center of the plot with maximum path difference at the edges. The carrier frequency of the fringe pattern is determined by the central wavenumber of the emission which is Doppler shifted by the wind speed. For a predominantly temperature broadened line, the width of the interferogram envelope is a measure of the temperature, with a higher temperature corresponding to a narrower envelope. The thick line in FIG. 1 illustrates the residual obtained by taking the difference between two interferograms each corresponding to a different wind speed, which causes them to have slightly different carrier frequencies. The thin curve shows the intensity vs. optical path difference for a Gaussian emission line as it would be recorded by a scanning Michelson interferometer scanned over the entire modulated path difference. Zero path difference is at the center of the plot where the visibility of the fringes is maximal. The maximum response of the measurement to wind speed is at path difference POPT where the amplitude of the signal difference is maximal. Assuming a temperature broadened, Gaussian line profile with width σD:
                              σ          D                =                              σ            0                    ⁢                                    kT                              mc                2                                                                        (        1        )            the optimum path difference is:
                              P          OPT                =                  1                      2            ⁢                          πσ              D                                                          (        2        )            where σ0 is the wavenumber of the line center, k is Boltzmann's constant, m is the molecular or atomic mass of the emission source, T is the temperature, and c is the speed of light.
Note that the fringe frequency in FIG. 1 has been greatly reduced for illustrative purposes. A real interferogram taken with a Michelson interferometer for a near infrared (NIR) emission line would produce ˜105 fringes between path differences 0 and POPT under typical atmospheric conditions.
Determining Doppler shifts with a phase-stepped Michelson requires the isolation of a single emission line with a pre-filter. A fit of the interferogram phase at the four measured samples is then possible, which can subsequently be used to determine the Doppler frequency shift. If the line is close to other emissions in the spectrum, the pre-filter has to be extremely narrow, which can be achieved by an additional Fabry-Perot etalon prefilter, with all of its attendant difficulties and the resulting reduction in throughput (see Ward, W. E. et al., “The Waves Michelson Interferometer: A visible/near IR interferometer for observing middle atmosphere dynamics and constituents,” Proc. SPIE Int. Soc. Opt. Eng., 4540, 100, 2001, and Ward, W. E. et al., “An imaging interferometer for satellite observation of wind and temperature on Mars, the Dynamics Atmosphere Mars Observer (DYNAMO),” Proc. SPIE Int. Soc. Opt. Eng., 4833, 226, 2002). Using an a priori line shape assumption (e.g. Gaussian or Voigt), the line width can be determined from the interferogram modulation, which yields the temperature for a predominantly temperature broadened line.
Several stepped FTS techniques have been used to measure Doppler shifts. The WINDII instrument uses piezoelectric actuators to move one mirror of the interferometer (Shepherd et al.). The MIMI (Mesospheric Imaging Michelson Interferometer) instrument uses a segmented mirror with four sections at different OPD, which avoids moving the mirror (Babcock et al.). The WAMI (Waves Michelson Interferometer) version, designed for the Earth's atmosphere, proposes a moving, segmented mirror, allowing the simultaneous measurement of two emission lines with a two step mirror scan (Ward et al. 1). A phase-stepped Michelson interferometer has also been proposed for Mars using a non-segmented, mirror moved by piezo actuators.
Disadvantages of FTS as discussed above include the need for moving parts (in case of a dynamically stepped system) and the reduced throughput due to the necessary pre-filter leading to an increase in the size and weight of the overall payload.
Spatial Heterodyne Spectroscopy Heritage
Spatial Heterodyne Spectroscopy (SHS) was conceived in the late 1980s and was mainly facilitated by the availability of array detectors (see Harlander J. M., R. J. Reynolds, and F. L. Roesler, “Spatial heterodyne spectroscopy for the exploration of diffuse interstellar emission lines at far ultraviolet wavelengths,” Astrophys. J., 396, 730-740, 1992, and Harlander J. M. et al., “Field-Widened Spatial Heterodyne Spectroscopy: Correcting for Optical Defects and New Vacuum Ultraviolet Performance Tests,” Proc. SPIE Int. Soc. Opt. Eng., 2280, 310, 1994). The basic principle of SHS is that the path difference that is typically scanned by a Michelson interferometer is imaged onto a position-sensitive detector without moving parts. This is accomplished by replacing the return mirrors in a Michelson interferometer with Littrow diffraction gratings and imaging the gratings onto the detector. SHS instruments measure all interferogram samples simultaneously in the spatial domain using a line or array detector. They heterodyne the spatial fringe frequency around the Littrow wavenumber, σL, of the gratings, which allows the optimum use of the number of array detector elements. As a result, SHS allows the design of compact, high throughput, high resolution spectrometers without moving parts. To date, SHS has mainly been used in the UV and visible. The first orbital flight of an SHS was performed in 2002 with the proof of concept mission of SHIMMER (Spatial Heterodyne Imager for Mesospheric Radicals) on the Space Shuttle (see Harlander J. M., F. L. Roesler, J. G. Cardon, C. R. Englert, and R. R. Conway, “SHIMMER: A Spatial Heterodyne Spectrometer for Remote Sensing of Earth's Middle Atmosphere,” Appl. Opt., 41, 1343-1352, 2002, Cardon J. G., C. R. Englert, J. M. Harlander, F. L. Roesler M. H. Stevens, “SHIMMER on STS-112: Development and Proof-of-Concept Flight, AIAA Space 2003 Conference & Exposition,” AIAA Paper 2003-6224, 2003, and Englert C. R., J. M. Harlander, J. G. Cardon, and F. L. Roesler, “Correction of phase distortion in spatial heterodyne spectroscopy,” Appl. Opt., 43, 6680-6687, 2004). An improved version of SHIMMER including a monolithic interferometer was placed in low-earth orbit on STPSat-1 in early 2007 (see Harlander J. M., F. L. Roesler, C. R. Englert, J. G. Cardon, R. R. Conway, C. M. Brown, J. Wimperis, “Robust monolithic ultraviolet interferometer for the SHIMMER instrument on STPSat-1,” Applied Optics, 42, 2829-2834, 2003).
A disadvantage of conventional SHS is its limited resolving power, which is typically not high enough to measure the Doppler shift caused by winds.
It would therefore be desirable to provide a system for wind measurements that is more robust and lighter in weight that present systems.